When, owing to an intake air quantity sensor failure, the detected intake air quantity includes an error, inappropriate fuel injection is performed. This leads to toxic substance release to the atmosphere.
For instance, Patent document 1 discloses a fault diagnosis apparatus configured to determine that an airflow sensor has a fault when an absolute value of the difference between the estimated intake air flow rate estimated by an intake air quantity estimation means and the actual intake air flow rate measured by the airflow sensor is greater than a value that the absolute value cannot usually take or a predetermined value G0 determined based on an engine rotation speed and engine load.
However, when the throttle opening becomes a low opening position or when the rotation speed of the internal combustion engine becomes low (i.e., in the operation range of low intake air quantities), even with the airflow meter unfailed, there is a risk that the actual intake air quantity greatly diverges in a direction for increasing or decreasing of a divergence value from the estimated intake air quantity owing to intake air pulsating flow.
That is, in the fault diagnosis apparatus for the airflow meter, as disclosed in Patent document 1, dispersions of the detected values of the airflow meter in the operation range of low intake air quantities, were not taken into account. For instance, suppose that the predetermined value G0 is set to a smaller value for the purpose of improving the fault-diagnosis accuracy. In this case, in the operation range of low intake air quantities, it is easy to cause a decision error (a diagnosis error). This leads to the problem that the airflow-meter fault diagnosis has to be halted in the operation range of low intake air quantities to prevent such a decision error.